Human multiple myeloma is a malignancy of hematopoietic stem cell origin which manifests itself at the mature stage of B-cell development. The present studies examine biological approaches to analyze the growth and differentiation of human myeloma. One human myeloma cell line, HS- Sultan, is growth inhibited by phorbol esters but not by bryostatin, a PKC activator currently being evaluated in clinical trials. In addition, HS-Sultan was observed to secrete interferon-gamma, a cytokine not previously reported to be produced by myeloma cells after phorbol ester treatment. Messenger RNA for TGF-beta has also been detected in this cell line and these studies are focused on determining the role of these secreted cytokines in the differentiation of this multiple myeloma cell line.